Any electronics people know a simple/cheap circuit to precisely measure current but have the measured side be dc isolated (it’s biased at 30V)?

my naïve thought is to just have a good shunt into a like difference circuit with an op amp and resistors and then an ADC can read that

is there a better way to do this?

edit (clarification): some folks picked up on this but I need to measure pretty small currents (<100uA) sensitively so magnetic sensors probably are out. thanks for all the ideas!
@xeno seems like a job for a wheatstone bridge